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I3 Theory:

Instigating, Impelling, and Inhibiting Factors in Aggression

Erica B. Slotter and Eli J. Finkel

Northwestern University

Interpersonal aggression is prevalent and disturbing. This chapter presents a meta-

theoretical perspective, I3 Theory, that seeks (a) to impose theoretical coherence on the massive

number of established risk factors for aggression and (b) to employ the tools of statistical (and

conceptual) moderation to gain new insights into the processes by which a previously

nonaggressive interaction escalates into an aggressive one (see Finkel, 2007, 2008). I3 Theory

(pronounced “I-Cubed Theory”) does not advance one key variable (or even a specific set of key

variables) as the root cause of aggression. Rather, it seeks to present an organizational structure

for understanding both (a) the process by which a given risk factor promotes aggression and (b)

how these risk factors interrelate to aggravate or mitigate the aggression-promoting tendencies of

each. As detailed below, I3 Theory suggests that scholars can predict whether an individual will

behave aggressively in a given situation by examining the main and interactive effects of the

Instigating triggers, Impelling forces, and Inhibiting forces at play.

Aggression refers to any behavior carried out with the primary proximal goal of inflicting

physical harm on a target who is motivated to avoid being harmed (Baron & Richardson, 1994).

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(We do not examine in this chapter other forms of aggression, such as verbal, relational, or

sexual aggression.) In a recent literature review, C. A. Anderson and Bushman (2002, p. 29)

report that “five main theories of aggression guide most current research”: (a) Cognitive

Neoassociation Theory, (b) Excitation Transfer Theory, (c) Social Learning Theory, (d) Script

Theory, and (e) Social Interaction Theory. In the ensuing paragraphs, we briefly review the core

tenets of each of these five theories and of a recent integration called the General Aggression

Model (C. A. Anderson & Bushman, 2002; DeWall & C. A. Anderson, Chapter 1, this volume).

Thereafter, we introduce I3 Theory and then briefly and selectively review the aggression

literature from its perspective.

Extant Theories of Aggression

According to Cognitive Neoassociation Theory (which largely subsumes the earlier

Frustration-Aggression Hypothesis; Dollard, Doob, Miller, Mowrer, & Sears, 1939), negative

affective states cause people to aggress (Berkowitz, 1993). Negative affect, which is frequently

caused by an aversive event such as frustration, provocation, or physical pain, can automatically

trigger cognitive, affective, physiological, and even motor tendencies associated with aggression.

Cognitive Neoassociation Theory also emphasizes that higher-level cognitive processes, such as

attributions and appraisals, can modulate these automatic “instigations to aggress.” According to

Excitation Transfer Theory (Zillman, 1983), misattributed physiological arousal leads to

aggression. If individuals are experiencing incidental (unrelated) physiological arousal when

they encounter an anger-inducing stimulus, their anger may become exacerbated by the

misattribution of the incidental arousal to this stimulus. This exacerbated anger can, in turn,

precipitate aggression.

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According to Social Learning Theory (Bandura, 1973), individuals develop aggressive

tendencies when they observe other individuals behave aggressively, particularly if those other

individuals are likable, have high status, are rewarded for their aggressive behavior, and so forth.

According to Script Theory (Huesmann, 1998; Huesmann, Dubow, & Boxer, Chapter 8, this

volume), individuals learn aggressive scripts from the world around them, including from the

mass media. Scripts, which refer to well-learned, highly accessible, and integrated mental

representations of situation-specific causal links, goals, and action plans (Schank & Abelson,

1977), can cause individuals to engage in complex aggressive acts once circumstances trigger a

key aspect of the aggressive script. Finally, according to Social Interaction Theory (Tedeschi &

Felson, 1994), individuals enact aggressive behavior to achieve instrumental goals, such as

obtaining something of value, exacting retribution for a perceived wrong, or establishing a

desired social identity. (See Shaver, Segev, & Mikulincer, Chapter 4, this volume, for another

functionalist perspective on aggression based on attachment theory and the concept of behavioral

systems.)

In an attempt to build a broad model of aggression encompassing the common features of

these theories, C. A. Anderson and Bushman (2002) honed their integrative, meta-theoretical

General Aggression Model. As with I3 Theory, the General Aggression Model focuses less on a

particular variable or process than on general classes of aggression risk factors and processes

(DeWall & C. A. Anderson, Chapter 1, this volume). This model consists of three main foci. The

first emphasizes person and situation inputs, or risk factors, for aggression. Person inputs include

personality traits, gender, beliefs, attitudes, values, long-term goals, and scripts; situation inputs

include aggressive cues (e.g., presence of guns), provocation, frustration, pain and discomfort,

drugs, and incentives (determined by a cost/benefit analysis). The second focus is the

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interconnected affective, arousal, and cognitive routes, or mechanisms, through which the inputs

influence aggressive behavior. Affective routes include mood and emotion and expressive motor

tendencies; arousal routes include the strengthening of a dominant action tendency or certain

misattribution processes; cognitive routes include hostile thoughts and scripts. Finally, the third

focus is the outcomes of the underlying appraisal and decision processes. Individuals are likely to

act impulsively when they lack the resources and motivation to alter their immediate appraisal of

the situation. If they possess the resources and motivation, however, they may reappraise the

situation and act in a more thoughtful fashion (DeWall & C. A. Anderson, Chapter 1, this

volume).

I3 Theory

I3 Theory, which is a process-oriented meta-theory designed to identify the circumstances

under which a nonaggressive interaction can become an aggressive one, builds upon these

previous theories, integrates them with recent research on self-regulation, and specifies the ways

in which aggression risk factors interact to predict aggressive behavior.

The theory begins by posing three questions. First, does at least one individual in the

interaction experience strong instigating triggers toward aggression? Second, does that individual

experience strong impelling forces toward aggression? Third, does that individual experience

weak forces to inhibit or override the aggressive impulses? Each affirmative answer increases the

likelihood of aggressive behavior, via both a main effect and interactive effects with variables

relevant to one or both of the other questions. Whereas the strength of impelling forces is

determined by the collective power of the variables that cause the individual to experience an

urge to aggress in response to a given instigating trigger, the strength of inhibiting forces is

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determined by the collective power of the variables that cause the individual to override this

aggressive urge.

In addition to the three initial questions posed above, I3 Theory poses a fourth: How do

effects of variables in one category (instigating triggers, impelling forces, or inhibiting forces)

interact with effects of one or more variables from the other categories to predict aggressive

behavior? As presented in Table 1, answering these four questions enables scholars to identify

seven key I3 Theory effects. Figure 1 (which builds upon work by Fals-Stewart, Leonard, &

Birchler, 2005) illustrates how these seven effects can work together to increase or decrease the

likelihood of aggressive behavior.

I3 Theory diverges from the aggression theories mentioned earlier in its central emphasis

on inhibitory processes. The theory recognizes the importance of instigating triggers and

impelling forces, but it argues that such factors cause individuals to enact aggressive behavior

only when their collective power is stronger than the collective power of inhibitory processes.

Although other theories address the importance of inhibitory processes in aggression (e.g.,

DeWall & C. A. Anderson, Chapter 1, this volume, as well as Denson, Chapter 6; Dodge,

Chapter 10; and Mayseless & Scharf, Chapter 14), the emphasis on such processes gains new

prominence with I3 Theory.

Instigating Triggers

The first stage of I3 Theory concerns the presence of one or more instigating triggers,

which are discrete situational events or circumstances that induce rudimentary action tendencies

toward physical aggression. As illustrated at the lower-left of Figure 1, impelling and inhibiting

forces are irrelevant when instigating triggers are absent. Even the world’s angriest, least

controlled person is not aggressive all the time; some situational variable (even if it only serves

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to activate a long-standing goal or grievance) is required before the person becomes aggressive.

Aversive events can trigger (via automatic associative networks or cognitive appraisal processes)

hostile cognitive, affective, physiological, and even motor tendencies that prime the individual to

aggress (Berkowitz, 1993; DeWall & C. A. Anderson, Chapter 1, and Dodge, Chapter 10, this

volume); I3 Theory suggests that certain events can also trigger aggressive tendencies driven by

instrumental goals (e.g., having an individual offer you money to beat up his enemy).

Instigating triggers fall into one of two categories: dyadic and third-party. Dyadic

triggers refer to events or circumstances that the potentially aggressive individual perceives as

having originated in the target. Examples include direct provocation (Bettencourt & Miller,

1996), goal obstruction (Dollard et al., 1939), and social rejection (Leary, Twenge, & Quinlivan,

2006). Third-party triggers refer to events or circumstances that the potentially aggressive

individual perceives as having originated in somebody other than the target. The same kind of

trigger that leads to a rudimentary action tendency to aggress toward the provocateur can also do

so toward a third party. For example, an individual who feels provoked or rejected may

experience an instigation to aggress not only against the source of the provocation or rejection,

but also (or alternatively) toward another target whom the potential perpetrator believes would

make a more acceptable or desirable target (e.g., somebody who is less likely to fight back).

Impelling Forces

The second stage of I3 Theory concerns risk factors that determine the strength of the

aggressive impulse experienced by the individual, either through main effects or through

interactions with instigating triggers. In some situations, individuals may effortlessly shrug off

(or perhaps not even notice; see Dodge, Chapter 10, this volume) an instigating trigger,

experiencing virtually no impulse toward aggression. In others, individuals may react strongly to

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a trigger, experiencing a powerful impulse toward aggression. Impelling forces refer to factors

that increase the likelihood that individuals will experience an aggressive impulse in response to

an instigating trigger. Individuals tend to experience more powerful aggressive impulses when

impelling forces are strong than when they are weak (see dashed vs. dotted lines in Figure 1),

especially to the degree that the instigating trigger is severe.

Impelling forces fall into one of four categories: evolutionary and cultural, personal,

dyadic, and situational. Evolutionary and cultural impellors refer to features of the potentially

aggressive individual’s biological or cultural heritage, including evolutionary adaptations and

social norms (see Cohen, Chapter 7, and Sell, Chapter 3, this volume). Examples include

adaptations resulting from evolutionary pressures that provided ancestral men and women with a

survival advantage for experiencing violent impulses in certain situations (Lorenz, 1966) and

social norms delineating the extent to which certain instigating triggers provoke strong

aggressive impulses (Nisbett & Cohen, 1996). Personal impellors refer to relatively stable

characteristics of a given individual that differ from those of many other individuals, including

personality characteristics, attitudes, beliefs, interpersonal interaction styles, or biological

factors. Examples include dispositional hostility (Norlander & Eckhardt, 2005; Rhee &

Waldman, Chapter 9, and Tackett & Krueger, Chapter 5, this volume), narcissism (Thomaes &

Bushman, Chapter 11, this volume; Twenge & Campbell, 2003), and testosterone (Dabbs, Frady,

Carr, & Besch, 1987; Van Goozen, Frijda, & Van de Poll, 1994). Dyadic impellors refer to

characteristics of the relationship between the potential aggressor and the potential target.

Examples include dissatisfaction with the amount of power one has in a relationship (Ronfeldt,

Kimerling, & Arias, 1998), target-specific jealousy (Dutton, van Ginkel, & Landolt, 1996;

Holtzworth-Munroe, Stuart, & Hutchinson, 1997), and feelings of vulnerability or insecurity in

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the relationship (Carney & Buttell, 2005; Holtzworth-Munroe et al., 1997; Mikulincer & Shaver,

Chapter 13, this volume). Finally, situational impellors refer to momentarily activated cognitive,

affective, or physiological experiences. Examples include uncomfortable temperatures (C. A.

Anderson, K. B. Anderson, Dorr, DeNeve, & Flanagan, 2000), physical pain (Berkowitz, 1998),

and exposure to violent media (C. A. Anderson & Bushman, 2001; C. A. Anderson, Carnagey, &

Eubanks, 2003).

Inhibiting Forces

The third stage of I3 Theory concerns risk factors that determine whether individuals will

override the aggressive impulses that emerge from the instigating triggers, impelling factors, and

their interaction. In some situations, individuals succumb to these impulses, engaging in

aggressive behavior. In others, individuals override them in favor of nonviolent behavior.

Inhibiting forces refer to factors that increase the likelihood that individuals will override

aggressive impulses rather than acting upon them. Inhibiting factors collectively determine the

threshold above which aggressive impulses will manifest themselves in aggressive behavior. If

the inhibiting forces are weak (lower horizontal line in Figure 1), then aggressive impulses need

not be especially strong to result in aggressive behavior. If the inhibiting forces are strong (upper

horizontal line in Figure 1), then aggressive impulses must be strong to result in aggressive

behavior.

As with impelling forces, inhibiting forces fall into one of four categories: evolutionary

and cultural, personal, dyadic, and situational. Examples of evolutionary and cultural inhibitors

include adaptations resulting from evolutionary pressures that provided ancestral men and

women with a survival advantage for overriding aggressive impulses in certain situations

(Baumeister, 2005; Sell, Chapter 3, this volume) and social norms or institutions that decrease

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the likelihood that individuals will act upon aggressive impulses (Cohen, Chapter 7, this volume;

Guerra, Huesmann, & Spindler, 2002; Sampson, Raudenbush, & Earls, 1997). Examples of

personal inhibitors include dispositional self-control (Finkel, DeWall, Slotter, Oaten, & Foshee,

in press), executive functioning (Denson, Chapter 6, this volume; Giancola, 2000), and beliefs

that enacting aggressive behavior will lead to poor outcomes for the self (Slaby & Guerra, 1988).

Examples of dyadic inhibitors include partner empathy or perspective-taking (Richardson,

Green, & Lago, 1998; Van Baardewijk, Stegge, Bushman, & Vermeiren, in press), relationship

commitment (Gaertner & Foshee, 1999; Slotter, Finkel, & Bodenhausen, 2009), and relative

physical size (Archer & Benson, 2008; Felson, 1996; Sell, Chapter 3, this volume). Finally,

examples of situational inhibitors include low levels of alcohol use (Bushman & Cooper, 1990;

Denson et al., 2008), self-regulatory resources that are not depleted (Denson, Chapter 6, this

volume; DeWall et al., 2007; Finkel et al., in press), and plentiful cognitive processing time

(Finkel et al., in press).

Reviewing the Aggression Literature from the Perspective of I3 Theory

One purpose of I3 Theory is to provide a coherent framework for categorizing aggression

risk factors and examining the interplay among them. To illustrate how I3 Theory can accomplish

these goals, we review key findings in the aggression literature through its lens, with a particular

emphasis on interaction effects. This review is illustrative rather than exhaustive.

I3 Theory encompasses seven key effects: three main effects (instigating triggers,

impelling forces, and inhibiting forces), three two-way interaction effects (Instigating Triggers ×

Impelling Forces, Instigating Triggers × Inhibiting Forces, and Impelling Forces × Inhibiting

Forces), and one three-way interaction effect (Instigating Triggers × Impelling Forces ×

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Inhibiting Forces). Table 1 lists these seven effects and provides an example of each one. We

discuss examples of these effects in turn.

1. An illustrative main effect of instigating triggers: Social rejection. As shown in the

first row of Table 1, an illustrative instigating trigger is social rejection, which refers to a class of

interpersonal processes in which individuals feel rejected, excluded, or ostracized. Diverse lines

of evidence demonstrate that individuals who experience social rejection are more aggressive

than individuals who do not (Leary et al., 2006). In one study, participants who had been

unanimously rejected by a group of fellow participants administered substantially louder, more

painful sound blasts to an unknown stranger than did participants who had been unanimously

accepted (Twenge, Baumeister, Tice, & Stucke, 2001, Study 5). In this study, social rejection

served as a displaced instigating trigger because the target of the aggression was not a member of

the group who had previously rejected the participant. Another study, which provided an in-

depth analysis of all well-documented school shootings in the United States between 1995 and

2001, yielded compatible conclusions, with acute or chronic rejection preceding the shootings in

87% of the cases (Leary, Kowalski, Smith, & Phillips, 2003).

2. An illustrative main effect of impelling forces: Testosterone. As shown in the second

row of Table 1, an illustrative impelling force is the androgen testosterone. Although

testosterone is higher in men than in women, it predicts aggression in both sexes (Archer,

Birring, & Wu, 1998; Dabbs & Hargrove, 1997; see Sapolsky, 1998). In a study of male prison

inmates, testosterone levels correlated positively with crime severity; indeed, nine of the 11

inmates with the lowest testosterone levels had committed nonviolent crimes, whereas 10 of the

11 inmates with the highest testosterone levels had committed violent crimes (Dabbs et al.,

1987). In another study, female-to-male transsexuals became considerably more aggressive in

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the first three months of androgen injections (Van Goozen, Cohen-Kettenis, Gooren, Frijda, &

Van de Poll, 1995).

3. An illustrative main effect of inhibiting forces: Self-regulatory strength. As shown in

the third row of Table 1, an illustrative inhibiting factor is self-regulatory strength, which refers

to the psychological resource that undergirds willful acts of self-regulation. According to the

Strength Model of self-regulation (Baumeister, Gailliot, DeWall, & Oaten, 2006), all such acts

depend on a unitary resource that resembles a muscle, in that its strength can be (a) temporarily

diminished by self-regulatory exertions (leading to a state of “ego depletion”) but (b) bolstered

over time by adhering to self-regulatory bolstering “regimens.” In one study, hungry participants

who had previously resisted eating a tempting food (a donut; high ego depletion condition) were

more aggressive toward a same-sex provoking interaction partner (forcing this partner to eat a

snack with plentiful hot sauce despite his or her distaste for spicy foods) than were participants

who had previously resisted eating a less tempting food (radishes; low ego depletion condition),

even though participants in the two conditions did not differ in how angry they were in response

to the provocation (DeWall et al., 2007, Study 1).

Complementing this evidence that ego depletion predicts elevated aggression, a recent

study demonstrated that individuals who had adhered to a two-week self-regulatory strength

bolstering regimen declined significantly from before to after the regimen in their aggressive

tendencies toward their romantic partner (Finkel et al., in press, Study 5). In this study,

participants who deliberately regulated either their physical behavior (e.g., brushing their teeth

with their nondominant hand) or their verbal behavior (e.g., making sure not to begin sentences

with “I”) reported a reduced likelihood of being physically aggressive in response to various

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partner provocations (e.g., “I walk in and catch my partner having sex with someone”), whereas

participants in a no-intervention control condition exhibited no change from pretest to posttest.

4. An illustrative Instigating Trigger × Impelling Forces interaction effect: Ego Threat ×

Narcissism. We now turn from of I3 Theory’s three main effects to its three two-way interaction

effects. As shown in the fourth row of Table 1, an illustrative Instigating Trigger × Impelling

Forces interaction effect is Ego Threat × Narcissism. Although many scholars have suggested

that low self-esteem causes aggression, others have increasingly argued that a form of high self-

esteem is frequently more likely to do so. In particular, individuals whose self-views are not only

favorable, but also unstable, inflated, or uncertain, are especially prone toward aggressive

behavior when their favorable self-views are socially threatened (Baumeister, Smart, & Boden,

1996; Thomaes & Bushman, Chapter 11, this volume). One series of studies demonstrated that

participants who had experienced an ego threat in the form of insulting feedback about an essay

they had written (an instigating trigger) were more aggressive toward the same-sex provocateur

(subjecting him or her to painfully loud noise blasts) than were participants who had not

experienced an ego threat (Bushman & Baumeister, 1998). The key finding, however, was that

this main effect of ego threat was substantially larger for participants who were high in

narcissism (an impelling factor) than for participants who were low.

Additional research has examined how self-views moderate the link between other

instigating triggers (aside from insults) and aggression. One study demonstrated that the link

between social rejection and displaced aggression (painful noise blasts) was substantially

stronger for individuals who were high in narcissism than for those who were low (Twenge &

Campbell, 2003, Study 4), and another demonstrated that the link between social rejection and

aggression (aversive hot sauce) was substantially stronger for individuals who were high in

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rejection sensitivity (who anxiously expect, readily perceive, and overreact to rejection) than for

those who were low (Ayduk, Gyurnak, & Luersson, 2008).

5. An illustrative Instigating Trigger × Inhibiting Forces interaction effect: Provocation

× Self-Regulatory Strength. As shown in the fifth row of Table 1, an illustrative Instigating

Trigger × Inhibiting Forces interaction effect is Provocation × Self-Regulatory Strength. As

discussed previously, both provocation and self-regulatory strength predict aggression via main

effects. Recent studies confirm the I3 Theory prediction that incorporating their interaction effect

yields a richer story (e.g., Denson, Chapter 6, this volume). Two recent experiments, one in

which the aggression was directed at strangers (aversive sound blasts; DeWall et al., 2007, Study

2) and one in which the aggression was directed at one’s romantic partner (forcing him or her to

maintain body poses for painfully long durations; Finkel et al., in press, Study 4), demonstrated

that participants were especially aggressive when they experienced both provocation (in the form

of insulting feedback) and ego depletion.

Another example of an Instigating Trigger × Inhibiting Forces interaction effect is

Provocation Severity × Frontal Lobe Functioning (Lau, Pihl, & Peterson, 1995). In this study,

participants were preselected if they were in the upper or lower quartile on frontal-lobe-based

cognitive functioning, which underlies the ability to control one’s impulses (Hecaen & Albert,

1978). Consistent with I3 Theory, participants were more aggressive (administering painful

electric shocks) to the degree that the opponent had previously provoked them severely rather

than mildly (i.e., had administered painful electric shocks to them), but this provocation main

effect was substantially stronger among individuals with weak rather than strong frontal lobe

functioning.

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Yet another example of an Instigating Trigger × Inhibiting Forces interaction effect is

Provocation Salience × Alcohol Consumption (Densen et al., 2008). In this study, participants

who were assigned to consume four alcoholic or placebo beverages in a 20-minute period were

provoked in either a salient or a subtle way and then had the opportunity to aggress against their

provocateur by determining for how long he or she would have to keep his or her hand immersed

in iced water. Participants were more aggressive in the salient provocation condition than in the

subtle provocation condition, and this effect was significantly stronger in the alcohol than in the

placebo condition.

A final example of an Instigating Trigger × Inhibiting Forces interaction effect is

Provocation Level × Relationship Commitment within the context of an ongoing romantic

relationship (Slotter et al., 2009). In these studies, participants were more aggressive toward their

romantic partner after they had been provoked by him or her, and this provocation main effect

was especially strong among participants who were low in relationship commitment (and even

dropped to nonsignificance among participants who were high). Extending work suggesting that

commitment promotes prorelationship behaviors in other conflictual relationship domains (such

as forgiveness; Finkel, Rusbult, Kumashiro, & Hannon, 2002), it appears that individuals who

are highly committed to their romantic relationships are able to override aggressive impulses

when their partner provokes them.

6. An illustrative Impelling Forces × Inhibiting Forces interaction effect: Physical

Proclivity × Negative Outcome Expectancies. As shown in the sixth row of Table 1, an

illustrative Impelling Forces × Inhibiting Forces interaction effect is Physical Proclivity ×

Negative Outcome Expectancies. Individuals vary in the degree to which they prefer physical

versus cognitive tasks, and a relative preference for the former predicts increased tendencies

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toward aggression and criminal behavior (Gottfredson & Hirschi, 1990). Individuals also vary in

the degree to which they believe that engaging in aggressive behavior will cause them to

experience negative outcomes, such as physical harm or social derision, and stronger beliefs in

this causal link predict decreased tendencies toward aggression (Slaby & Guerra, 1988). One

recent study examined whether physical proclivity (an impelling factor) interacts with negative

outcome expectancies (an inhibiting factor) to predict aggression toward a romantic partner

(Finkel & Foshee, 2009). Results revealed a strong positive association of physical proclivity

with self-reported aggressive behavior over the previous year, but only when negative outcome

expectancies were low. It seems that the tendencies to prefer physical to cognitive tasks predicts

greater aggression among individuals whose aggression is not restrained by beliefs that being

aggressive will bring about negative effects for them, but not among individuals whose

aggression is so restrained.

7. Instigating Triggers × Impelling Forces × Inhibiting Forces interaction effects: A null

set. Although the three main effects and the three two-way interaction effects reviewed above are

key components of I3 Theory, the Instigating Triggers × Impelling Forces × Inhibiting Forces

three-way interaction effect arguably represents the most conceptually important component of

the theory. Indeed, the theory suggests that all three of the preceding two-way interaction effects

are moderated by third variables from whichever category is not represented in that two-way

interaction.

Testing for such a three-way interaction would not be difficult. For example, it would be

easy to examine (a) whether the Ego Threat × Narcissism (Instigating Trigger × Impelling

Factor) interaction effect is moderated by an inhibiting factor (e.g., self-regulatory strength,

alcohol consumption, strong relationship commitment), (b) whether the Provocation × Self-

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Regulation Strength (Instigating Trigger × Inhibiting Factor) interaction effect is moderated by

an impelling factor (e.g., testosterone, physical proclivity, dispositional anger), and (c) whether

the Physical Proclivity × Negative Outcome Expectancy (Impelling Factor × Inhibiting Factor)

interaction effect is moderated by an instigating trigger (e.g., social rejection, ego threat,

provocation). Testing for such three-way interaction effects is an important direction for future

research.

Discussion

I3 Theory seeks (a) to impose enhanced theoretical coherence on the vast array of

aggression risk factors by identifying how each of them increases the likelihood of aggression

(via instigating triggers, impelling forces, and/or inhibiting forces) and (b) to examine the

manner in which risk factors from one category interface with those from one or both of the

other categories to predict aggressive behavior. In the preceding discussion (also see Table 1 and

Figure 1), we reviewed specific examples of how certain risk factors fit into I3 Theory and how

they interface with variables from the other I3 Theory categories.

One important direction for future research (aside from providing the first tests of I3

Theory’s Instigating Trigger × Impelling Factor × Inhibiting Factor three-way interaction effect)

will be to develop and hone empirical procedures for classifying a given risk factor into an I3

Theory category (instigating trigger, impelling forces, and/or inhibiting forces)—or perhaps into

more than one category if a given variable both increases aggressive impulses and decreases

restraint. In this chapter, we have relied on theory to determine, for example, (a) that social

rejection, ego threat, and provocation, are instigating triggers; (b) that testosterone, narcissism,

and physical proclivity are impelling factors; and (c) that self-regulatory strength, lack of alcohol

consumption, and negative outcome expectancies are inhibiting factors. One limitation of this

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approach is that existing theory is in many cases not sufficiently developed vis-à-vis the I3

Theory parameters to allow for definitive classification. For example, we are reasonably

confident that dispositional self-control predicts reduced aggression in large part by raising the

inhibition threshold (see horizontal lines in Figure 1), thereby increasing the likelihood that

individuals will override aggressive impulses. That said, perhaps such dispositional self-control

also predicts reduced aggression in part by reducing impelling forces, thereby decreasing the

strength of the aggressive impulse in the first place.

How might scholars employ empirical procedures to determine whether a given variable

promotes aggression by increasing aggressive impulses or by decreasing restraint (or the

proportion of the variance attributable to each mechanism)? One promising approach is to adapt

recent developments in process dissociation paradigms used by social cognition researchers.

Scholars have recently modeled behavior on laboratory tasks to discern the degree to which

participants exhibit certain automatic tendencies (e.g., toward prejudice or discrimination) and

also controlled tendencies that override these automatic tendencies (Payne, 2001; Sherman et al.,

2008; see Jacoby, 1991). After developing empirical procedures for distinguishing impulses

toward aggressive behavior from self-controlled processes that override those impulses, scholars

will be able to examine the association of a given risk factor with both (a) individuals’ tendencies

to experience impulses to aggress and (b) their tendencies to override those impulses (Denson,

Chapter 6, this volume). We predict that variables such as testosterone, narcissism, and physical

proclivity will correlate positively with the automatic aggressive tendencies identified by these

process dissociation procedures and negligibly with the controlled tendencies that override these

automatic tendencies. In contrast, we predict that variables such as self-regulatory strength,

sobriety, and negative outcome expectancies will correlate negligibly with the automatic

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aggressive tendencies identified by these process dissociation procedures and positively with the

controlled tendencies that override these automatic tendencies.

Once scholars determine (using theoretical tools, empirical tools, or both) which risk

factors function by strengthening aggressive impulses and which function by weakening

behavioral override processes, I3 Theory may hold promise for interventions designed to reduce

aggressive behavior. For example, the theory highlights the importance of inhibitory factors

(especially self-regulation) in predicting aggressive behavior, and scholars have suggested that

interventions designed to help individuals override their impulses are likely to be more effective

than interventions designed to prevent them from experiencing those impulses in the first place

(e.g., Baumeister, 2005). To the degree that such scholars are correct, interventions designed to

strengthen inhibitory forces may turn out to be more effective on average than interventions

designed to weaken impelling forces. Early research inspired by I3 Theory suggests that

inhibition-relevant interventions at the dispositional level (e.g., building self-regulatory strength

via bolstering regimens), relational level (e.g., strengthening relationship commitment), and

situational level (e.g., having participants wait 10 seconds before responding to a provocation)

can reduce aggressive behavior (Finkel et al., in press; Slotter et al., 2009; see Finkel et al., 2002,

for evidence that relationship commitment is amenable to experimental manipulation). And, as

discussed above, distal inhibitory factors such as prevalent social norms also seem to influence

aggressive behavior (Cohen, Chapter 7, this volume; Guerra, Huesmann, & Spindler, 2002;

Sampson, Raudenbush, & Earls, 1997), which hints at the possibility that large-scale social

interventions could potentially reduce aggression at the societal level.

Shifting back to the theoretical domain, I3 Theory is broadly consistent with all of the

major theories reviewed above, agreeing that aggression is predicted by negative affect

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(Berkowitz, 1993), misattributed physiological arousal (Zillman, 1983), social learning of

aggressive conduct (Bandura, 1973), aggressive scripts (Huesmann et al., Chapter 8, this

volume), and certain instrumental goals (Tedeschi & Felson, 1994). I3 Theory seeks to establish

whether (and the degree to which) these processes promote aggression by strengthening

aggressive impulses or by weakening inhibitions against aggression (e.g., misattributed

physiological arousal may be more likely to predict aggressive behavior by bolstering aggressive

impulses than by impairing restraint processes). To be sure, scholars in these established

theoretical traditions have discussed related issues (e.g., Berkowitz, 1993); I3 Theory seeks to

elevate this discussion to a primary emphasis, while simultaneously linking ideas across theories.

In conclusion, I3 Theory is an attempt to categorize aggression risk factors into instigating

triggers, impelling factors, and/or inhibiting factors and to identify the interplay among variables

across categories (see Figure 1). To the degree that extant theory provides good reason to believe

that particular risk factors fit relatively neatly into one of the I3 Theory categories, the theory

provides an immediately accessible agenda for future research, oriented less toward identifying

additional risk factors than toward identifying (a) the processes by which risk factors, considered

in isolation, increase aggression; and (b) the manner in which they interact to do so. In the long-

run, I3 Theory can inform interventions designed to help individuals manage their aggressive

impulses in a constructive manner. Indeed, scholarship may well progress to the point where

interventions can be tailored to the specific inhibiting risk factors most relevant to a given

person, perhaps reducing one individual’s aggression by bolstering self-regulatory strength and

reducing another individual’s aggression by bolstering empathy. Such tailored interventions,

which would require valid assessment instruments, hold particular promise for reducing the

prevalence and severity of interpersonal aggression.

20

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27

Table 1

I3 Theory of Aggression: The Seven Effects

I3 Effect (#)

I3 Effect (Stage)

I3 Effect (Description)

Example

Citation for Example

1

Stage 1

Instigating Trigger main effect

Social Rejection

Twenge et al., 2001

2

Stage 2

Impelling Forces main effect

Testosterone

Dabbs et al., 1987

3

Stage 3

Inhibiting Forces main effect

Self-Regulatory Strength

DeW

all et al., 2007

4

Stage 1 × Stage 2

Instigating Trigger × Impelling

Ego Threat × Narcissism

Bushman & Baumeister,

Forces Interaction Effect

1998

5

Stage 1 × Stage 3

Instigating Trigger × Inhibiting

Provocation × Self-Regulatory

Finkel et al., in press

Forces Interaction Effect

Strength

6

Stage 2 × Stage 3

Impelling Forces × Inhibiting

Physical Proclivity × Negative

Finkel & Foshee, 2009

Forces Interaction Effect

Outcome Expectancies

7

Stage 1 × Stage 2 × Instigating Trigger × Impelling

[No Known Example]

[No Known Example]

Stage 3

Forces × Inhibiting Forces

Interaction Effect

28

Figure 1. H

ow

the

thre

e co

mponen

ts o

f I3

Theo

ry inte

rrel

ate

to p

redic

t aggre

ssiv

e beh

avi

or.

Intensity of Violent Impulses

Strong Impelling Forces

Weak Impelling Forces

Absent Moderate Extreme

Intensity of Instigating Triggers

Violence Threshold

Given Weak

Inhibiting Forces

Violence Threshold

Given Strong

Inhibiting Forces

Note. For ease of illustration, Figure 1 depicts impelling forces and inhibiting forces as if they

are binary—either weak or strong. In reality, the intensity of each type of force varies

continuously from weak to strong.